The long-term objective of our research is to understand the computations performed by cerebral cortical circuit. Toward this end, we focus on understanding the circuitry of cat primary visual cortex (VI) and the manner in which it produces the functional response properties of cat VI neurons. We focus on can VI because it is by far the best-studied piece of cerebral cortex, and because it is part of the process of visual perception which is the sensory modality that is best understood at the cortical level. In particular, we aim to understand the functional connectivity of cat V1, that is, which neurons are connected to one another by excitatory or inhibitory synapses, as a function of the visual response properties, cortical layers of origin, and excitatory or inhibitory nature of the neurons. This will be accomplished by simultaneously recording from multiple nearby neurons using the tetrode method of recording, which allows the simultaneous isolation of multiple neurons at single recording sites. We will use cross-correlation analysis to infer which of the simultaneously recorded neurons make monosynaptic connections to one another and the sign of the connection when it exists. Evidence of an inhibitory connection simultaneously provides evidence of the excitatory of inhibitory nature, respectively, of the presynaptic neuron. We will functionally characterize the recorded neurons using a combination of traditional grating stimuli and noise stimuli. We will determine the cortical layers in which recorded neurons are located through histological analysis after the conclusion of the experiment. By studying large numbers of pairs, and determining who is connected to whom, vs. cell layers functional response properties, and excitatory or inhibitory nature of the connection, we will build up a statistical picture of the functional connectivity of cat V1. This information, in interaction with modeling of the cortical circuit, provides the basis for understanding how the function of cat V1 is created from its circuit structure. Understanding of cortical function in turn provides the basis for understanding visual disorders such as amblyopia and strabismus and neurological disorders due to stroke, and more generally for understanding normal function and its disorders such as learning disabilities.